Orthodontic treatment involves movement of malpositioned teeth to orthodontically correct positions. Tiny orthodontic appliances known as brackets are connected to exterior surfaces of the patient's teeth, and an archwire is placed in a slot of each bracket. The archwire forms a track to guide movement of the teeth to desired positions for correct occlusion. End sections of the archwire are often received in appliances known as buccal tubes that are fixed to the patient's molar teeth. In recent years it has become common practice to use adhesives to bond orthodontic appliances to the surface of the tooth, using either direct or indirect methods.
For many years, it was common practice to apply orthodontic adhesive to the base of the appliance immediately before the appliance was placed on the tooth. In some instances, a quantity of adhesive was dispensed onto a mixing pad or dispensing well, and a small spatula or other hand instrument was then used to apply a small dab of adhesive to each appliance. In other instances, a quantity of adhesive was dispensed from a syringe directly onto the base of the appliance.
Adhesive precoated brackets are known and offer significant advantages to the orthodontist. Adhesive precoated brackets have a bonding base upon which the manufacturer may apply a precise quantity of adhesive such as a photocurable adhesive. In direct bonding methods, when it is desired to mount the bracket on a tooth, the bracket is simply removed from the package and placed directly onto the tooth. For indirect bonding methods, brackets are removed from the package and placed, for example, on a replica plaster or “stone” model of the patient's dental arch to provide a custom base for later mounting on the patient's tooth, commonly using a transfer tray.
In addition to good handling properties and adequate adhesion, practitioners often desire that photocurable adhesives provide other desirable functional properties, including, for example, sustained fluoride release. In many applications, it is also desirable that the adhesive have adequate hydrophilicity or moisture tolerance, as indicated, for example, by the ability of the adhesive to absorb water or saliva, and/or the ability to provide an adequate bond under moist conditions.
Orthodontic brackets precoated with an adhesive are commonly sold with a flexible release substrate in contact with the adhesive. However, many release substrates are not suitable for use with all adhesives. For example, when soft, tacky, less viscous, hydrophilic adhesives are used, it has been found that a portion of the adhesive is sometimes left on the release substrate when an attempt is made to lift the appliance from the container and detach the adhesive from the release substrate. In those instances, sufficient adhesive may not remain on the bracket to provide adequate bond strength to retain the bracket on the tooth during the course of orthodontic treatment. Moreover, adhesives having a relatively low viscosity tend to slowly flow across the release substrate and from the space beneath the bracket over extended periods of time, creating bracket removal and/or liner release problems.
Additionally, when soft, tacky, less viscous adhesives are used with conventional adhesive precoated appliance packages, the shape of the adhesive may distort as the appliance is lifted from the container. In some instances, detaching the adhesive from the release substrate may alter the configuration of the adhesive, resulting in unsatisfactory direct bonding unless additional steps are undertaken to shift the adhesive by hand back to its original, pillow-like shape. For example, when lifting an appliance from the container, some of the adhesive may be shifted to one side of the appliance base, such that the opposite side of the base does not have a satisfactory amount of adhesive for bonding. If the appliance is secured to a tooth structure in such a manner that a void space is present between a portion of the base and the opposed tooth surface, the void space may result in premature, spontaneous debonding of the appliance from the tooth, a nuisance that is best avoided. Furthermore, in some instances the void space can establish a pocket that receives food and debris, facilitating the formation of caries.
Presently, there is a need in the art for adhesives and methods that provide a balance of properties and characteristics that are desired by the practitioner.